Commercially available adhesive can be divided into four categories based on their composition: hot melt adhesives, reaction resin adhesives, adhesives that are based on aqueous dispersions and adhesives that are based on solvents.
Hot melt adhesives are generally applied by means of a conventional extrusion and coating technique at temperatures between 100 and 300° C. onto one of two surfaces to be attached by gluing. The other, “cold” surface is then brought into contact with the coated surface, whereby the melt is cooled off and becomes hardened so that a permanent bond is created between the surfaces that are to be glued to each other. Hot melt adhesives are often used in the packing industry and they are used for the manufacturing of self-adhesive layers.
Reaction resin adhesives are characterized by the fact that the polymers enabling the adhesion and the bond are first formed in situ after the application of the adhesive from monomers or from prepolymers. Reaction resin adhesives usually harden very quickly. They are therefore well suited for quick and permanent bonding of contacts which have a small surface area. Products which are in demand in private practice, such as products called “instant adhesives” and “all-purpose adhesives”, which are based on methacrylate or cyanoacrylate, belong to this group.
On the other hand, other characteristics are required from adhesives for applications in internal construction. Processing of these adhesive should be easy, it should be possible to process them or have them available for a long period of time, and they should be suitable for application in a wide range of temperatures; furthermore, they should display good initial adhesion characteristics, a wide adhesive spectrum for application to different types of substrate and they should also guarantee a permanent bond lasting for decades. Moreover, in the case of floor adhesives, it is also important that the health risks, which are caused when a person breathes in volatile organic components of adhesives during the processing of adhesives, should be kept as low as possible.
In order to protect the processing personnel, dispersion adhesives are the preferred class. Unfortunately, dispersion adhesives have a number of disadvantages, which result from their composition. The emulsifier encloses relatively long chains of polymers, and this emulsifier is then dispersed again in water. The wetting of the substrate is often insufficient due to a high surface tension of the water and a high polarity. The high evaporation enthalpy of the water means that a long time period is needed for drying of the adhesive. The penetration of the polymer into the surface of the substrate is low due to the size of the dispersed polymer particles. The hardened adhesive film is sensitive to moisture due to the remaining emulsifier.
Adhesives that are based on solvents have the following advantages when compared to adhesives that are based on aqueous dispersions: Due to the clearly lower evaporation enthalpy of the solvents, the adhesives dry much faster (only approximately ⅓ of the energy amount will be required). The low surface tension and the low polarity of the solvents improve the wetting of the substrate. The solvents dissolve a number of substrates and the adhesion is thus improved, in particular on dense, smooth substrates. The polymers are dissolved as isolated chains and therefore penetrate very well into porous substrates. The hardened adhesive film is much less sensitive to moisture.
Reaction resin adhesives play only a minor role in applications performed by skilled workers due to protection of the health of the personnel handling the adhesive. Problems are caused when reactive monomers or prepolymers come into contact with skin, or when reactive, volatile compounds penetrate into the respiratory tract.
Hot melt adhesives cannot be easily handled on a large surface area by skilled workers, which is why they are used only in internal construction and on small surface areas in special cases.
In spite of the technical advantages which are displayed by solvent adhesives, their application is limited. This is due to issues relating to environmental protection, fire protection, and most importantly, protection of the processing personnel. Solvent-based adhesives are employed mainly in the production of adhesive tapes, for gluing foam substances, for gluing of wood flooring, and in the form of contact adhesives.
The task of the present invention is therefore to provide a novel adhesive composition based on solvents which is in compliance with the requirement for protection of the labor force and the environment, in particular in compliance with the upper limit value for organic compounds in air according to TRGS 900 [TRGS=Technical Rules for Dangerous Substances] and the EPA, and which at the same time maintains the advantageous characteristics of solvent-based adhesives.
In accordance with the invention, this task is achieved with an adhesive composition whose main constituents are based on organic solvents acetone and ethanol, using a thermoplastic polymer, preferably polyvinyl acetate, and a hydroxyl-modified thermoplastic resin. The remaining components are additives and additional substances known from prior art. Preferred additives comprise plasticizers and fillers, particularly preferred are siliceous and carbonate fillers, which meet varying technical requirements thanks to their flexible chemical and physical composition, as well as mixing ratios. The adhesive according to the invention can also contain other additives such as rheological additives (for example castor oil derivatives, polyamides), wetting agents, antioxidants or pigments. The composition preferably comprises the following amounts of processed end products:                1 to 12 weight % of acetone, preferably 4 to 8 weight %,        4 to 16 weight % of ethanol, preferably 9 to 13 weight %,        5 to 12 weight % of polyvinyl acetate, preferably 8 to 10 weight %,        10 to 25 weight % of a hydroxyl-modified hydrocarbon resin, preferably 15 to 19 weight %,        0.5 to 5 weight % of plasticizers, preferably 1 to 3 weight %,        35 to 65 weight % of fillers, preferably 45 to 60 weight %,        0 to 5 weight % of additives, preferably 0 to 2.5 weight %        
An advantage of the adhesive according to the invention is the fact that solvent emissions, which occur during the processing, are significantly below the current maximum concentration valid for work places (according to TRGS 900) of 960 mg of ethanol per cubic meter of air and of 1,200 mg of acetone for cubic meter of air. This adhesive will be now explained in more detail on preferred embodiment of the adhesive according to the invention. The following is preferably used for the composition of the adhesive:                5 weight % of acetone,        12 weight % of ethanol        8.5 weight % of polyvinyl acetate resin (VINNAPAS B 60, UW 1, UW 10, Wacker Polymer Systems, Burghausen), comprised of a mixture of three polymers which have a median molecular weight of 74,000 g/mol, 150,000 g/mol and 435,000 g/mol, and which are contained with a relative content amount to each other of 5:5:7, wherein the resin with the highest molecular weight also has the highest weight percentage (the molecular weights are determined from the limiting viscosity number η).        16.7 weight % of a phenol-modified cumaron-indene resin (Novares CA 90 made by the Ruttgers Chemicals, Duisburg), with a median molecular weight of about 1,000 g/mol (Mw).        1.9 weight % of the softener diisobutyl phthalate        6 weight % of kaolin        49.9 weight % of CaCO3 comprising two grading curves for amorphous chalk and one grading curve for finely crushed crystalline marble        